The present invention relates to filtering systems and, in particular, it concerns a backwash arrangement for cleaning a cylindrical filter screen.
Large throughput filtration systems are used for treating liquids (for example, water or sea water) in a wide range of applications. The present invention relates particularly to filtering systems in which liquids are passed through one or more cylindrical filter screen, and in which the inner surface of the filter screen is cleaned of accumulated debris by a backwash arrangement that generates low pressure in a suction nozzle passing over the inside of the filter screen. Debris accumulated on the internal surface of the filter screen during filtration is lifted out of the filter screen by a flow of the filtered liquid in a reverse direction (“backwash”) through the screen into the nozzle, from where it is expelled from the system. Examples of such systems are described in the following patent documents: DE 19944107, GB 1485989, US 2008/0047885 and WO 2011/058556.
For an optimal cleaning process, liquid would be drawn through the screen without leakage of unfiltered liquid from around the nozzle into the suction arrangement. Liquid sucked from the surroundings of the nozzle instead of through the filter screen reduces efficacy of the self-cleaning process, and results in significant wastage of energy and of liquid. The main factor that determines the ratio between filtered liquid backwashed through the filter and liquid leaking from around the nozzle is the proximity of the suction nozzle to the filter screen. It is therefore desirable to bring the suction nozzle as close as possible to the screen. The filter screens however are fine, and have significant variations (tolerances) from an exact cylindrical shape, making it difficult to reduce a clearance between the backwash nozzles and the screen.
In an attempt to achieve close proximity of the nozzles to the screen, some of the above references suggest the use of spring-loaded nozzles that press against the screen. This however increases friction and wear of the components, thereby reducing the lifetime of the system.
There is therefore a need for a backwash arrangement for cleaning a cylindrical filter screen which would achieve close positioning of the backwash nozzles to the screen without generating greatly increased wear and friction moment.